Optical assessment of neural activity typically requires an invasive detection system. For example, axons may be coated with nanometer-diameter gold particles to assess neural activity by measuring rapid changes in axon diameter using a laser interferometer. Differences in reflected or back-reflected light from these particles may then be detected by the interferometer. Rapid mechanical changes may also be assessed using a piezoceramic bender attached to the axon and a stylus. Similarly, an optical lever may be configured such that one edge rests on an axon and an opposing edge rests on a fixed point (e.g., a knife edge). Swelling or shrinkage of the axon may be assessed by detecting changes in reflection of incident light. In addition, slow changes (on the order of a few seconds to a few minutes) in neural activity may be detected using optical coherence tomography, but this technique is insensitive to rapid changes (on the order of 20 milliseconds) and small changes (on the order of nanometers).
Due, at least in part, to their invasiveness, the above-described techniques are typically not useful in a clinical setting.